Hydrogen Bonds in Coal——The Influence of Coal Rank and the Recognition of a New Hydrogen Bond in Coa

CHEM. RES. CHINESE U. 2003. 19(1),70-75Hydrogen bonds in coalThe Influence of Coal Rank and the recognitionof a New Hydrogen bond in CoalLI Dong-tao, LI Wen"*and lI Bao-qingState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of SciencesTaiyuan 030001, P.R. ChinaReceived febBy means of in-situ diffuse reflectance FTIR, the IR spectra of 6 coals with different ranks were obtainedfrom room temperature to 230 C. A new curve fitting method was used to recognize the different hydrogenbonds in the coals, and the influence of coal ranks on the distribution of hydrogen bonds(HBs in the coalsand their thermal stability were discussed. The results show that there is another new HB(around 2514cm )between the SH in mercaptans or thiophenols and the nitrogen in the pyridine-like compounds in thecoals,and the evidence for that was provided. The controversial band of the HB bet ween hydroxyl and the nitrogen of the pyridine-likedetermined in the range of 3028--2984 cm, and the result is consistent with but more specific than that of Painter et aL.. It was found that the stability of different HBs inthe coals is influenced by both coal rank and temperature. For some HBs, the higher the coal rank, the higher the stability of them. Within the temperature range of our research, the stability of the hB between thhydroxyl and the r bond increases to some extent for some coals at temperatures higher than 110 or 140 CKeywords Hydrogen bonding, Thermal stability, Coal rank, Coal, In-situ diffuse reflectance FTIRArtic|e|D1005-9040(2003)-01-070-06Introductionvent swelling have been used in the study of hydro-It is well known that hydrogen bonding in coal gen bonding in coal. Among them, IR technique isaffects the properties, the chemical structure and the most widely used method for this kind of rethe conversion of coal significantly. Larsen n] has search because of its unique advantages. The inestimated that the number of such intersegment frared study of hydrogen bonding in coal can besecondary-bonds is four times that of covalent classified into three stages 5. First, the evidencecross-links. BrennerL2I has pointed out that hydro- for the existence of hydrogen bonding in coal isgen bonding probably accounts for the high T' and mainly concerned 6-9. Second, the band assign-glassy properties of coal. Larsen et al. [3 think that ments of the infrared spectroscopy absorbance ofhydrogen boning in coal is closely related with the each hydrogen bond in coal are carried Out (6.10ck-likeness and brittleness of coal. minter et alco-workers[4) found that after preheatment, the strong impact on the later researches. USing trans-cross linking of hydrogen bonding in coal was sup- mittance IR, Chen et al. studied the hydrogenpressed, and hence the conversion of coal in- bonding in coal and asphaltenes and found fivecreased. Moreover, hydrogen bonding in coal also types of hydrogen bonds. Third, the quantitativeplays an important role in the swelling, the pyroly- study of hydrogen bonds in coal z is made. Bysis and the reactivity of coal. It is extremely impor- means of curve fitting, the spectrum ranging fromtant to study the hydrogen bonding in coal for bet- 3700 to 2400 cm was divided into 10 peaks, andter understanding the structure and effective uti- son中国煤化工 esults were obtainedlization of ceitu diffuse reflectanceCNMHUp to now, several methods such as infra-red FTlCillliyuu and a new peak resolvingspectroscopy (IR), difference scanning calorimetry method were used to study the influence of coal(DSC), thermogravimetric analysis(TGA) and sol- rank on the hydrogen bonds in the coals. MoreSupisnts*E the National Natural Science Foundation of China(No. 29906012)* To whom correspondence should be addressedLI Dong-tao et al.71over, a new hydrogen bond in the coals was ob- tra, the number and the position of the subpeaks inserved, that is, the hydrogen bond formed by the this range were determined. It was found thathesH in the mercaptans or the thiophenol 9nitrogen atom in the pyridine-like compoundsthere were 14 peaks in the range of 3700--2000cm, and their assignments are given in TableExperimentalhydrogen bond (SHN) formed by the nitrogen atom in the pyridine-like compounds and theSix coals with different carbon contents wereSH in mercaptans or thiophenols was found forused, and the ultimate and the proximate analyses the first time, and the evidence for the existence ofof them are listed in Table 1. All the samples(100 it will be given in the part of"results and discusmesh) were further ground in an agate mortar unsIonder argon for 30 min in a glove boTable 2 The band assignments of the subpeakTable 1 Analysis of samplesnd their positionsProximate analysis(%) Ultimate analysis(%, daf.The barnments of subpeaks Positions of subpeaks, v/cmSample v(daf. M (ad )A(ad) C H N S O(diff. Minerals(kaolinite, illite etc.)3694±3,3654±5,3624±3XD41,322961,506.600.973.84361139.6118.452.4268.665.022.240.3023.78OH-π hydrogen bonds3560±11HLH48.4217.3825.5673.114.701.220.4420.53Self- associated n-mers(n>3)3406±10Y241.421.543.7479.305.041.193.0411.43 OH-ether O hydrogen bondsST 22.56 1.00 17.08 89. 14 4. 95 1.57 0.46 3.88 Tightly bound cyclic OH tetramers3182+1811.631.0714.6191.093.571.313.001.0Aromatic C-H3058±122 Apparatus and procedureOH-N(acid/base structure3006±22The diffuse reflectance spectra of all the samAliphatic C-H2966±5,2925士2,2860±5ples were measured on an EQUINOX 55 FTIRCooH dimers2739±15Hydrogen bond between the--SH ofspectrometer (Bruker) with the 0030-102 high tem- mercaptans or thiophenols and the ni/highZnsetrogen of pyridine-like compoundfrom Company Thermo Spectra-Tech. The detecThe adsorption of free Oh groups was not found in our retor was liquid-nitrogen-cooled MCT(mercury cad- search, but it is still necessary to give the band assignment of it.mium telluride). A mirror that is believed not toAs far as the current peak resolving methodbsorb water was used as the background 12. The for the study of hydrogen bonds in the coal is condiffuse reflectance spectra were based on the co-ad- cerned, it is practically impossible to determine thedition of 200 scans with a resolution of 4 cminitial half-width of each subpeak for curve fittingThe resultal spectra were converted to Kubelka- because of the serious overlapping of the peaksMunk functionFortunately, we can get many useful data on theAbout 20 mg of parent coal sample was filled half-widths of most of the hydrogen bonds coninto the reactor, and leveled with a spatula. As cerned from literatures[13-15]. But for the peakssoon as the dome of the reactor was fixed, argon not obviously overlapped with others, the initialwas introduced at a rate of 260 mL/min. After the half-widths of them for peak resolving are deterequilibration of the moisture of the sample with ar- mined directly through the second derivative spec-gon was carried out for about 90 min, the spectrum tra. The initial half-width of the hydrogen bond ofat room temperature was recorded. The spectra at SHN was measured from the spectra presented indifferent temperatures were measured after 5 min- literature[16_utes stabilization for the samples at 50, 80, 110In general, there are three models of the shape140, 170, 200 and 230 C respectively, and the for peak resolving, i, e, Gaussian, Lorentzian, orheating rate between adjacent temperatures was中国煤化工it was proved bbout 20 C/min-n distribution was pre3 Curve FittingCNMHGolving of coal. Hence,With the OPUS/IR software supplied by the we adopted the Gaussian distribution in our workspectrometer, the spectra between 3700--2000 and found it was very effectivee peak-resolved. The concrete procedureTheoretically speaking, to improve the reliais as follow方数据bility and the accuracy of the results of peak resolvudged from the second derivative of the spec- ing, it should be presented more parametCHEM. RES. CHINESE UVol. 19this procedure will become more complicated and where Ai; is the apparent intensity of a certain HBHtly. After several optat a certain temperature j and PA i is the corremizations, the parameters such as the peak posi- sponding adjusting parameter for ittion, the half-width and the shape of the sub-peaksIt is impossible to obtain the real amount ofwere presented, and the intensities of them were each HBH with equation(3), but for our discus-the only variable parameters for our peak resolv- sion, this treatment is enough and convenient(ining. Considering that CH bonds(both aliphatic fact, we can know the real amount of them by us-and aromatic )are hard to decompose and to form ing the chemical functional group analysis orbelow 300 C, the infrared absorbance intensities of I C NMR combined with peak resolving ). As forthem were also presented respectively. This the dimmers of carboxylic acids and SH-N, themethod was proved to be successful, and as an ex- apparent intensities of them were used directly forample, the result of peak resolving for XD coal is comparison. Because, for one thing, it is not ahown in Fig. 1simple matter to obtain the absorptivity of themfor another. therebond for each, and it is meaningless to discuss thedistribution of them20Results and discussion310I The evidence for the existence of sh-NUp to now, it has been proved that there are36003200280024002000six types of HBs including five HBHs and the hydrogen bonds of carboxylic acid dimmers in coalHowever, when we studied the infrared spectra ofFig. 1 The curve resolving of the spectra of XDthe coals with different sulfur and nitrogen con4 Calculationstents,it was found that the IR spectra of coals havFrom peak resolving, only apparent intensity ing a high rank with a high sulfur content displayed(integral area)of each sub-peak was obtained. Ac- shoulder peaks near 2514 cm( Fig. 2 ). The seccording to the result of Miura et aL. L12, the absorp- ond derivative spectra of them displayed negativetivity of the hydrogen bonded OH(HBH) follows peaks at this position, which cannot be assigned toequation (1)the contribution of noise(Fig 3)MOH CoHo (1+0.0147AVOH)In view of this, we think that there is a newhere oh is the absorptivity of HBh and ao o is hydrogen bond in the coals. It is the hydrogenthe absorptivity of free OH, Avo is the OH wave bond formed by theSH in mercaptannumber shift relative to the position of free OH on phenols and the nitrogen in the pyridine-like com-the forming of hydrogen bondspounds(SH--N). The evidence is as the followingn this paper, 3611 cm -IL10 is used as the ab(1) The spectra in Figs. 2 and 3 give a power-sorption position of free OH. Then the absorptivity ful evidence. Moreover, when we studied the IRof each HBH relative to that of free Oh is given by spectra obtained by Painter 1O and Solomon 7,weequation(2), and it is named as the adjusting pa- found that this peak also appeared in their spectrameter(PA). Because of the difference of the ab- Unfortunately, probably because of the limitationsorption position of each HBH in various coals, thealue of Pa calculated is consequently differentfrom each otherPA= CoH/aoHo CoH o(1 +0. 0147AvoH )/aoH o中国煤化工+0.0147△v(2)CNMHThe meanings of the parameters in equation2514cm-1(2)are the same as those in equation (1)36004002000hen the relative amount (x i) of hbh can becalculated数据 uation(3):A,/PA,=A;/(1+0.0147△H)(3)Fig. 2 The spectra of coals with different sulfur contentsLI Dong-tao et al.73the- SH in the thiophenols or the mercaptans andthe nitrogen in the pyridine-like compounds, whichhas not been found in coal before272925142 The Determination of the absorbance of hB Between the Nitrogen Atom of Pyridine-like Com2960pounds and the Hydroxyl Group(n-Oh)ofN—HO,dif36003200280024002000ferent researchers gave various assignmentsPainter et aL. 10 assigned it in the range betweenFig 3 The second derivative of YZ coal3100-2800 cm based on the model compoundsf the type of the coal used in their research, thiChen et al. L assigned it at 3150 cm based on thepeak was not noted by them although its existenceIR study of coals and asphaltenes. Miura et al. 12Jis undeniable if we study their spectra more carefulconsidered the peak of 2940 cm as the position oflyN-HO in coal for their peak resolvi(2)Using IR, Gordy et al. 16) studied the hysearch, we studied the second derivative spectra ofdrogen bonds between mercaptans or thiphenolsthe six coals at different temperatures(48 spectra inand pyridine, a-picoline and dibenzylamine, and total) and found that there was always a strongcame to the conclusion that thiophenols form hyshoulder near the aromatic C-H vibrationaldrogen bonds with the substances of a rather narstretching of 3060 cm in the range of 3028-2984row range of basicities. For the substances with cm(Fig 3). This peak cannot be attributed tobasicity constants much lower than that of pyrithe contribution of noise because noise occurs randine, no appreciable interaction occurred. For thedomlynd wont appearsubstances with basicities much stronger than that assigned this range to the absorbance of the hydro-of pyridine, the thiophenols appeared to be comgen bond of N-HO. This result is consistent withpletely ionized by the base to form a salt. Although but more accurate than that of Painter et al. [ioythe aliphatic mercaptans have low acidity con3 Influence of Coal Rank on the Distribution ofstants, and a low tendency to share their proton inHBHhydrogen bonds formation, the evidence for the hy-It has been proved that even if the coal weredrogen bonding bet ween them and the pyridine-likepreheated at a high temperature, it would still abcompounds is certainly undoubtedsorb moisture in high purity nitrogen during coolIns(3) According to the basic knowledge of coalhe parent coal weresciencells, about 50%-75% nitrogen exists in thepredried under vacuum, it would contact air andforms of pyridine or quinoline, and the rest existssorb the moisture in it during the filling of thein the forms of amino, imino, cyan and five-memsample into the reactor. Fortunately, Miura etbered ring organic nitrogen compounds. On the12 has found that the absorbed moisture by thother hand. the sulfur in coal exists in the formscoal can be completely removed at 140 C, and thesuch as thiophenols, mercaptans, thio-ether, and influence of adsorbed water on the hydrogen bondhiophene etc.. Hence, it is not strange that coal is ing in coal is eliminated. Therefore, it is possibleable to form the hydrogen bond of SH--Nto know the influence of coal rank on the distribution of hBhs in coal by means of thespectra inrangeis rarely affected by other absorbancethe spectra, at 140 C, of each coal with different(5) The absence of this shoulder in the IR ranks by in-situ Ftir technique.heco i e yH中国煤化工mounts of all hbhs00 %, then the percentCNMHGained. and the plots ofand consequently the strong absorbance of carboxylic acid dimmers in this range covers the weakthe hbhs contents versus the carbon contents ofabsorbance of sh—Nthe coals are given in Fig 4. Clearly, for low rankIn view of the above evidence, we suggest that coals such as XD, XF, the hydrogen bond of self-there is a Ftw*d of hydrogen bonds formed by associated OH n-mers is the main HB type, andsecondly the hydrogen bond of oh-t. The order ofCHEM. RES. CHINESE UVol. 19steric conditions for the formation of oh-t. whenonly a small amount of hydroxyl groups is presenin coal. and the steric condition allows. the Ohwill prefer to form other HBHs instead of (H-Tand the amount of it decreases consequently. Asfor almost the same contents for Oh- and self associated OH n-mers at some carbon percentage, it61.5068.6673.1179.3089.1491.09is completely a coincidenceon content(%, daf)4 The Influence of Coal Rank on the Thermal staFig, 4 The influence of coal rank on the distribution ofbility of HBHsHBHsIn Fig. 5, the sum of apparent inteself associated OH: 6. OH-: c, cyclic tetramgral area) of the various HBh for a certaincoal atd. OH-N: e, OH-OR.room temperature is set to be 100%, and the dethe other three ones: tightly bound cyclic OH crease ratio(negative value) for them can be calcitetramers>OH-N>OH-OR2(OH-ether O hy- lated respectively. Then the plots of the ratios ver-drogen bond ). With the increase of coal rank, the sus temperatures for the six coals can be obtainedcontent of OH- increases significantly to the maxi- and the influence of coal rank on the thermal stabilmum at the carbon content of 89 %. For high rank ity of each HBH is discussed. It must be pointedignite and bituminous coals, its content is almost out that because of the influence of moisture inthe same as that of self-associated OH n-mers, and coal, the regularities at low temperatures is not ob-simultaneously the order for other HBHs changes vious until the temperature is higgher than 110 orand becomes OH-N>tightly bound cyclic Oh te- 140 Chigher rank coalFrom Fig. 5. it is clear that various HBHscontent of tightly bound cOh tetramers dehave different thermal stabilities the thermal stacreases quickly, and becomes the least HBH. Con- bility of tightly bound cyclic Oh tetramers decreastrary to it, the content of n-HO increases great- es quickly with the increase of coal rank. It is inly. It seems that the higher nitrogen contents of st teresting that the variation of oh-N is just oppoand ZB than those of Hlh and YZ can account for site to that of tightly bound cyclic tetramers. Itsthis phenomenon. But after further examination of stability increases so quickly that the amount of itboth the nitrogen content and the trend of n-HO, cannot be significantly affected in the temperatureit was found that this possibility is very small. range of our research even at the carbon content ofMaybe it is more reasonable to relate it with89%. OH- is rather stable at the temperaturesvariation of coal chemical structure of differehigher than 110 or 140 C, and the amount of it inranKscreases at higher temperatures for some coals, andThe variation of each HBH reveals the change the enhancing comprehensive spectra, around 3 540of coal chemical structure with the increase of cm, of these coals also sustain this point. As forrank. For the low rank coals, more small molecu- the self-associated OH n-mers, the stability oflar substances(e. g, the derivatives of phenols them improves as the coal rank increases with someexist, so the self-associated HB is predominant. At exception. The dependence of OH--OR, on coalthe same time, because the low rank coals have al- rank is not obviousready some aromaticity, the amount of OH- is al- 5 The Thermal Stability of COOH Dimmers ando more than that of the other types of HBHs.With the increase of coal rank, both the increase ofas that inaromaticity and the decrease of hydroxyl中国煤化工dimmers and sh--Ncontent in coal occur. consequently resulting in theCNMH GIV the thermal stabili-quick increasing of OH- with a maximum at the ty of sh-N is weak in a low rank coal. At temcarbon content of 89%.peratures higher than 110 C, it becomes very unAs for the sharp decrease of OH- amount stable. In the temperature range of 110-230C,itwhen the fit*content is higher than 89%, an- follows the law that the higher the rank of a coalother factor must be considered, that is, the harsh the higher the stability of SH--NLI Dong-tao et al.750.40.5-0.5C)2050801101401702002302050801101401702002205080110140170200230-0.40.4205080110140170200230205080110140170200230205080110140170200230Fig. 5 The thermal stabilities of HBH(A) XD:(B) HLH: (C)ST:(D)XF:(E)YZ:(F) ZBa. OH-OR2: b. OH-N: c. cyclic hydroxyl tetramers; d. self-associated hydroxyl; e.OH-7with Prof. Dong Qing-nian in our instituteReferences[1 Larsen J. W, Am. Chem. Soc. Diu. Fue[2] Brenner d.,Fel,1985,64,1670.31985,50,4729205080110140170200230[4 Miura K, Mae K, Sakurada K, et al., Energy & Fuels[5 Li D, Li W.. Li B. Chemistry, 2001.7.4116 Petersen J. C, Fuel, 1967,46. 297] Taylor S.R.,LiN.C.,Fel,1978,57(2),117[8 Solomon P. R, Carangelo R. M., Fuel. 1988, 67.949[91 Fuller E. L. Jr, Smryl N. R, Fuel, 1985,64, 1143L10 Painter P. C, Sobkowiak M., Youtcheff ], Fuel. 1987080110140170200230[11] Chen C, Xu X, Gao J ., et al., Energy &Fuels, 1998℃12,446[12 Miura K., Mae K., Wen L, et al., Energy Fuels,200115(3),599[13 Huggins C. M, Pimentel G. C, Phys Chem.,1956Fig.6 The thermal stabilities of cooh dimers60,1615and s[14 Cairns T, Eglinton Dr. G., Nature, 1962, 196(4854),535A)Dimer of carboxylic acid; (B)SH--N.[15 Pimentel G. C, McClellan, The Hydrogen Bond, FreemanW. H. Company, Sanfrancisco and London. 1960.67For-COOH dimmers, the stability is more [16] Gordy W. Stanford S, Journal ofAmerican Chenmical sweak than that of-SH--N. The relationship beiety,1940,62(1),497tween its stability and coal rank is中国煤化工G. Carangelo r.M.,Col2 Characterization Techniqueof SH-N though the regularity is not soar o that hCNMHGSymposium Series, AmericanChemical Society, Washington D. C,1982. 205,77[18 Gueao, Coal Chemistry, Chemical Industry Press,AcknowledgementThe序数据 preciate the valuable discussion